It is the objective of the proposed research to investigate the transfer of hydrophilic materials through the placenta. The methods used will include physiological transfer measurement conducted with radio isotopes and electron microscopic investigation of steady state diffusion gradients, and the use of electron microscopic information to correlate structure with function. It is further envisaged to study the individual contributions of the histological layers of the hemochorial placenta to the total resistance of diffusion in the expectation that this information will enable us to extrapolate from animal experiments to human placental function, thus making unnecessary detailed physiological investigation of the normal human placenta in patients. Transplacental electrical potentials which may contribute, or be accompanied by active transport of electrolytes will be investigated with standard electrophysiological techniques.